In connection with forming sheet blanks, such as sheets of steel or aluminium for use in vehicles, it is known to use so-called diaphragm presses of the kind described above. In such a press, a forming tool is provided in a closed press chamber and the sheet blank to be formed is arranged between the forming tool and an elastic diaphragm of rubber or a polymer, which divides the press chamber in two separate chamber portions in a gas and liquid tight manner. By supplying a liquid at high pressure at the opposite side of the diaphragm, the diaphragm will be deformed and press the sheet blank against the forming tool whereby the blank obtains the desired shape. Forming using diaphragm presses of this kind is particularly advantageous in conjunction with relatively small production runs, since the cost of manufacturing the forming tools is relatively low.
However, in forming operations using a diaphragm press, as in every other kind of forming operation, it is desirable that, to the utmost possible extent, the blank be formed into a finished product in one single forming step, thereby avoiding, as far as possible, any subsequent machining that would increase the costs. Thus, it is desirable that, in the forming operation, the blank be cut to the correct size and shape and that, if desired, flanged edges be formed, said flanged edges being used, for example, for attaching the finished blank and for connecting it to other structural elements of the end product, such as a vehicle.
EP 0 288 705 discloses a forming tool for diaphragm presses by means of which it is possible to cut the sheet-metal blank to the desired size and to form a flanged edge along the periphery of the cut sheet-metal blank. This is achieved by the forming tool having cutting bars with a sharp cutting edge along the circumference of the forming tool and recesses for forming the flanged edges after cutting. During the forming process, the sheet-metal blank will, at a certain degree of deformation, be pressed against the cutting edges, which will cause the sheet to rupture and be cut off to the predetermined size, flanged edges being then formed at the cut-off edge portions by said portions being pressed into the recesses provided therefor in the forming tool. One drawback of this forming tool is, however, that the sheet-metal blank has to be deformed over a very long distance before it reaches the cutting edge and is cut off, which has a negative effect on the dimensional accuracy and other quality aspects. In addition, it is often desirable to prestretch the sheet-metal blank in one direction to the yield point of the material, preferably 3–6% beyond the yield point, before the forming operation in order to avoid elastic springback of the sheet-metal blank after forming. This implies a lateral contraction of the sheet-metal blank in the other direction, and in the case where there are large recesses, for example for the purpose of forming flanged edges, parallel to the stretching direction, the sheet-metal blank will be deformed by said recesses, which results in a product of inferior quality.